1. Field of the Invention
The present invention relates to a solid-state imaging apparatus for reducing the preservation time for one screen in a solid-state imaging device and providing a photoelectrically-converted output.
2. Description of the Related Art
In recent years, an imaging apparatus using a solid-state imaging device such as a charge-coupled device (hereinafter, CCD) instead of an image pickup tube has become a mainstream. The solid-state imaging apparatus has various superb features the imaging apparatus using an image pickup tube does not have. Currently, an increase in the number of pixels determining resolution is contradictory to improvement of the sensitivity in photoelectric conversion. For instance, when an image sensor is used with the number of pixels increased but with the chip size unchanged, the area of the image sensor that can undergo photoelectric conversion becomes smaller. Consequently, the sensitivity deteriorates.
An apparatus for resolving the above drawback; that is, for extending a saturation limit for photoelectric conversion in a solid-state imaging device, and thus expanding a dynamic range has been disclosed in Japanese Patent Application No. 63-98286 and Japanese Patent Laid-Open No.61-244759. In this conventional imaging apparatus, the preservation time for one screen in the solid-state imaging device is reduced to one of multiple time intervals (1/n field period, where n denotes an integer) corresponding to multiple divisions of a one-field period in a television signal mode, and thus pixel outputs of the solid-state imaging device are transferred at a high speed. The pixel outputs transferred at a high speed are stored screen by screen in a memory means. The pixel outputs stored screen by screen in the memory means are then added up for multiple screens. The time bases of the pixel outputs are then converted into the one for the foregoing one-field period in the television signal mode. Finally, an output resulting from the addition is read using the converted time base.
According to the aforesaid arrangement, preservation of a single screen (or field) in the solid-state imaging device is achieved for a shorter period of time than the time for scanning one field in the normal television signal mode. The saturation limit in the solid-state device is therefore extended. For reading, pixel outputs for multiple screens (fields) are added up. The time bases of the pixel outputs are then converted into the one for one field in the normal television mode. Consequently, thermal noises and reading noises are minimized. This results in an improved signal-to-noise ratio and an expanded dynamic range.
In the conventional apparatus, preservation of charges for one screen (one field) in the solid-state imaging device is achieved for a shorter period of time than a one-field time in the normal television signal mode. During reading, however, since pixel outputs for multiple screens (fields) are added up, an exposure time or a shutter speed becomes equal to the one-field time in the normal television signal mode. Specifically, when a subject moving faster than the one-field time is photographed, the image is blurred to deteriorate the image quality.
In efforts to prevent blurring of an image, the employment of an electronic shutter for varying the time of storing charges in the solid-state imaging device is conceivable. When a fast-moving subject is imaged, since the preservation time becomes shorter, the sensitivity deteriorates. To compensate for this deterioration, signals are amplified electrically. This results in a poor signal-to-noise ratio and a complex structure of an apparatus.